Increased activity of carotid sinus baroreceptors by sympathetic stimulation and norepinephrine

Effects of electrical stimulation of sympathetic nerves to the carotid sinus on the discharge of single active baroreceptor fibers of the rabbit were examined in situ and in functionally isolated carotid sinus preparations with an intact sympathetic innervation under controlled conditions of pressure and temperature. Among 30 single units, 18 units responded to sympathetic stimulation of increasing discharge frequency. The excitatory effect of sympathetic stimulation on baroreceptor activity was not abolished by phentolamine (1 mg/kg iv or 10(-6) g/ml in perfusate). In isolated carotid sinus preparations perfused with Krebs-Henseleit solution, various pressure steps were applied to the sinus, and effects of norepinephrine (NE; 10(-9) and 10(-6) g/ml) on activity of nine single baroreceptor units were examined. In the presence of 10(-9) g/ml NE, discharge frequency of all units significantly increased at a given pressure step when compared with the control, whereas NE at a high concentration (10(-6) g/ml) did not produce significant changes in the discharge frequency. It is concluded that NE released by sympathetic nerve endings most likely acts directly on the baroreceptor nerve endings and sensitizes them.